This invention relates to a process and apparatus for improving the separation of fluid cracking catalyst particles from gaseous hydrocarbons and stripping of hydrocarbons from the catalyst. More particularly, the invention is concerned with improving the separation of catalyst particles from gaseous hydrocarbon conversion products from a riser reactor in a catalytic cracking process and improving the stripping of hydrocarbons from the separated catalysts.
Dehne, U.S. Pat. No. 3,802,570 and Giles, U.S. Pat. No. 4,212,653 describe cyclone separators which include vortex stabilizing means for improving separation efficiency.
Anderson, et al., U.S. Pat. No. 4,043,899 and Heffley et al., U.K. Pat. App. No. 2,013,530 A, describe cyclones which have been modified to include a separate cyclonic stripping of catalyst separated from hydrocarbon vapors from a riser cracker.
Dries, U.S. patent application Ser. No. 4,313,910 describes an apparatus for separating a carrier gas from a particle stream from a riser reactor by deflecting the particle stream about a curved surface and introducing a fluid such as steam to rapidly disengage hydrocarbons from the catalyst particles.
The use of zeolite cracking catalysts, requiring short, fixed reaction times, has substantially affected catalytic cracking process design during the last few years. Modern catalytic cracking technology uses riser reactors, with rapid solid-vapor disengaging at the riser exit. In this process, the traditional reactor vessel has been relegated to the role of solids disengaging (i.e., gravity settler). Several designs are commercially available to treat the unique reaction engineering problems associated with fast-fluidized riser reactors. Numerous problems, regarding vapor/catalyst disengaging, remain to be solved. Several methods and means for solving these problems are proposed in the above-described patents which are incorporated herein by reference. However, none of the above describe the present invention.
The benefits of good feed/catalyst contacting include greater gasoline yield, a less pronounced catalyst density gradient (radially), faster feed vaporization, lower gas mix, and generally improved operability. As "ultimate" yields in zeolitic cracking are approached, engineering and hardware limitations will most certainly govern operations and the cracking reaction scheme itself.
Effectively terminating cracking reactions at the riser exit to accrue benefits of increased gasoline make, decreased gas make, and a more olefinic product requires stripping of interstitial and adsorbed hydrocarbons in the vapor/solids disengaging device. Increased gasoline yields result from a reduction in excessive secondary reactions, which occur if the hydrocarbons remain in contact with the catalyst beyond a desirably short reaction time. Those catalytic cracking units with gas compressor throughput capacity limitations will immediately benefit from the reduced gas production resulting from rapid vapor/catalyst disengaging and quick stripping of interstitial hydrocarbon. Reduced amounts of entrained and adsorbed hydrocarbons going into the regenerator will benefit units which are "coke" burning limited.
The hydrocarbons which must be separated from the catalyst include the bulk product vapor, the interstitial vapor, and the adsorbed products. The bulk product vapor is that which is separated, quickly and easily, by mechanical means (cyclones). The interstitial vapor can be displaced, relatively rapidly, by "stripping" gas, preferably steam. The adsorbed product requires a longer time to desorb and requires additional steam stripping. It is an object of this invention to provide mechanical disengaging/quick stripping to separate the bulk and interstitial vapors from the cracking catalyst as rapidly as possible. This minimizes overcracking and reduces the amount of carbon deposited on the catalyst. It is a further object of this invention to provide additional stripping to remove adsorbed products as rapidly as possible.
Heretofore, the introduction of stripping gas into a cyclone separator resulted in a loss of separation efficiency and was impractical.
It has now been found that when vortex stabilizing means are incorporated in a cyclone separator, stripping gas can be added to the cyclone separation zone without substantial loss of efficiency. Consequently, the cyclone separator and downstream stripper may be combined to achieve the concomitant benefits of quick stripping to remove bulk product vapor and interstitial vapor and to provide the longer stripping time required to desorb adsorbed hydrocarbon products from the catalyst.